This study reports the effect of Ba0.8Ca0.2TiO3 (BCT) nanoparticles incorporation on the structural, thermal, optical, and dielectric properties of the nanocomposite films with a polyvinyl alcohol (PVA) polymer matrix. To develop high-performance dielectric materials, we report for the first time a novel strategy for elaborating stable aqueous nanosized BCT suspensions by using a polyelectrolyte (PAA–NH4) dispersant whose concentration was about 2.0 wt% in a basic medium, which was thereafter dispersed in PVA under ultrasonication, which has demonstrated its ability to perform uniform PVA/BCT suspensions. The obtained materials were then deposited onto the substrate by using a spin coating technique to finally obtain BCT/PVA nanocomposites. The BCT weight fraction was in the range of 1–5 wt%. The X-ray diffraction analysis of BCT revealed a tetragonal phase with a P4mm space group. Furthermore, the XRD analysis of BCT/PVA nanocomposites emphasized a significant alteration of the PVA structure owing to the interfacial interaction between the polymer matrix and the BCT nanofillers. The micrographs of the nanocomposites confirmed the very good dispersion of BCT nanoparticles in the PVA polymer matrix. Thermogravimetric analyses of pure and doped PVA were performed in order to determine the effect of BCT on the thermal stability of ceramic-polymer nanocomposite films. Likewise, differential scanning calorimetry analysis showed that the incorporation of BCT nanoparticles had little influence on the glass transition temperatures (Tg) of different samples. The ultraviolet–visible spectroscopy revealed that the gap energy of the nanocomposites decreased as the BCT nanoparticles concentration increased in the polymer matrix. It went from 4.18 eV for pure PVA to 4.11 eV for 5wt%. Moreover, the addition of BCT nanoparticles into the PVA matrix has raised the dielectric constant over 3.84 times, reaching 7.99 for PVA/BCT-5wt% at 1 kHz at room temperature. Therefore, these PVA/BCT nanocomposites are potential flexible, high-performance dielectric materials for electronic devices.